Several methods for obtaining the corresponding nitrile from an aldoxime using a dehydrating agent have been known. Typical examples of the methods include (A) a method in which an aldoxime is dehydrated by a reaction with an acid anhydride such as acetic anhydride, (B) a method in which an aldoxime is dehydrated with phosphonitrilic chloride (hexachlorocyclotriphosphazatriene), and (C) a method in which an aldoxime is dehydrated by a reaction with a basic compound such as an alkali metal hydroxide (WO93/02046).
However, the employment of the above-described method (A) is disadvantageous in that acetic anhydride has to be used in an equimolar amount or more to the aldoxime, and hence the starting material cost is high. In addition, the acetic anhydride is finally converted to acetic acid, and hence the waste acetic acid treatment is necessary. The employment of the method (B) is also disadvantageous in that phosphonitrilic chloride has to be used in an equimolar amount or more relative to the aldoxime, and hence the starting material cost increases, and moreover that the waste product treatment cost is not lower in this case than in the case of acetic anhydride. The employment of the method (C) is disadvantageous in that the produced nitrile compound may be converted by hydrolysis to the amide compound depending on the reaction temperature or the reaction time (see Comparative Examples 1 to 3). In this respect, a method in which an acid such as sulfuric acid is used as the dehydrating agent has been reported as a method for overcoming these disadvantages (Japanese Patent Application Publication No. Sho 58-096053). This method is better than the above-described methods, because it is only necessary to use a catalytic amount of the acid, and the amount of waste products is small. However, this method is disadvantageous because of its limited application range. Specifically, nitriles can be obtained from aromatic oximes in yields of 80% or higher, but the yields of nitriles from aliphatic oximes decrease to 70%. Hence, this method cannot be practically applied to a wide range.